1. Field of the Invention
The present invention relates to input detection circuits, and in particular, to an input detection circuit for use with a matrix keypad or keyboard of a touch tone telephone.
2. Description of the Prior Art
A conventional telephone keypad that is used in a touch tone mode is configured in the form of a matrix having a plurality of rows R1-R4 and a plurality of columns C1-C4 of matrix pins. Referring to FIG. 1, resistors connect certain rows and columns to allow the selection of various dialing specifications or options. The conventional keypad of FIG. 1 operates in the following manner. When the user picks up the telephone handset or receiver, the circuit will enter the "scan option" mode in which it scans or detects the selected options represented by the specific resistor connections. These options can include: (1) the blockage of calls made outside a certain area code, (2) operation of the telephone in a pulse dial mode, and (3) the requirement to dial a "9" to get an outside line, among others. The scan option mode is performed very quickly, typically within 20 msec. After the scan option mode has been completed, the telephone enters the "press key" mode in which it waits for the user to press certain keys which the circuit will recognize as representing the telephone number to be dialed.
Unfortunately, the conventional keypad suffers from several disadvantages. First, if a key is pressed when the options are being scanned (i.e., during the scan option mode), errors may result. For example, if a user presses a key before lifting the handset, the circuit may detect an unknown option, or may override a previously programmed option. Second, since the conventional keypad circuit utilizes resistors to connect rows and columns, during stand-by time (i.e., the waiting time before and between key presses), the differences in the potential of the resistors will cause current to pass through the resistors, resulting in power dissipation.
Thus, there still remains a need for an input detection circuit for a matrix keypad which overcomes the drawbacks of the prior art circuits, which provides an improved input detection circuit, which prevents errors during the scanning of options, and which minimizes the loss of power during use.